Although the metering and control system of the present invention has general application, as will be evident as the description proceeds, it finds particular application as a vacuum gage controller.
Present-day vacuum gages use a thermo couple junction which is heated by a separate filament heater, with the heat from the heater passing through the vacuum environment whose pressure is to be measured. Since the thermal conductivity of gases varies significantly with vacuum pressure in the range, for example, of 0.1-1,000 microns, variations in pressure within that range produce significant changes in the temperature of the thermo-couple junction, and significant changes in its voltage output. In accordance with the prior art procedures, an electric meter is connected to the thermo couple which responds to the voltage output of the thermo couple, and which may be calibrated directly in pressure (0.1-1,000 microns) to function as a vacuum pressure gage.
The prior art systems also include controllers coupled to the meter, so that various control effects may be instituted as the meter passes through selected adjustable set points. In such prior art systems, it is usual to provide a relay for each set point, and which is activated when the indicating needle of the meter passes through the particular set point.
The prior art controllers are usually coupled to the meters by mechanical or optical means. However, such prior art controllers are subject to certain drawbacks in that the mechanical means are limited in their accuracy and the optical means are relatively complex and expensive.
The system of the present invention provides a separate solid state control circuit for controlling each relay as the indicating needle of the meter passes through corresponding adjustable set points. The control circuits function independently of the meter during normal operation, and the overall system provides a simple, inexpensive and precise means for activating the relays as the meter passes through the corresponding set points.
As mentioned above, the metering and control system of the invention finds particular, although not exclusive, utility in conjunction with vacuum equipment requiring pressure monitoring, automatic pressure control and/or safety interlocks. Such pressure control may include, for example, the sequencing of valves, the protection and control of diffusion pumps, and/or the actuation of alarm devices and pressure controls.